Referring to FIG. 1, a conventional radio frequency (RF) power amplifier includes a Wilkinson power divider 81, two power amplifier modules 82 and a Wilkinson power combiner 83.
The Wilkinson power divider 81 includes an input terminal 811 and two output terminals 812, 813. An equivalent impedance (Z91) seen into the Wilkinson power divider 81 from the input terminal 811 thereof is 50 ohms. Equivalent impedances (Z94, Z95) seen into the Wilkinson power divider 81 respectively from the output terminals 812, 813 thereof are both 50 ohms. The Wilkinson power divider 81 receives a radio frequency input signal with a power of P1 at the input terminal 811 thereof, divides the radio frequency input signal into two first radio frequency signals, each with a power of P2, and outputs the first radio frequency signals respectively at the output terminals 812, 813 thereof.
Each power amplifier module 82 is coupled to a respective output terminal 812, 813 of the Wilkinson power divider 81 for receiving a respective first radio frequency signal therefrom, and amplifies the power of the respective first radio frequency signal by a predetermined amplification factor to obtain a respective second radio frequency signal with a power of P3.
The Wilkinson power combiner 83 includes two input terminals 831, 832 coupled respectively to the power amplifier modules 82, and an output terminal 833. Equivalent input impedances (Z92, Z93) seen into the Wilkinson power combiner 83 respectively from the input terminals 831, 832 thereof are both 50 ohms. An equivalent output impedance (Z96) seen into the Wilkinson power combiner 83 from the output terminal 833 thereof is 50 ohms. The Wilkinson power combiner 83 receives the second radio frequency signals respectively from the power amplifier modules 82, combines the second radio frequency signals to obtain a radio frequency output signal with a power of P4, and outputs the radio frequency output signal at the output terminal 833 thereof.
The Wilkinson power divider 81 achieves a high degree of isolation between the output terminals 812, 813 thereof (i.e., a scattering parameter therebetween is zero) when the equivalent impedances seen into the Wilkinson power divider 81 respectively from the input and output terminals 811, 812, 813 thereof are matched. The Wilkinson power combiner 83 achieves a high degree of isolation between the input terminals 831, 832 thereof (i.e., a scattering parameter therebetween is zero) when the equivalent impedances seen into the Wilkinson power combiner 83 respectively from the input and output terminals 831, 832, 833 thereof are matched. In order to achieve impedance matching, the Wilkinson power divider 81 has to include a resistive element coupled between the output terminals 812, 813 thereof, and the Wilkinson power combiner 83 has to include a resistive element coupled between the input terminals 831, 832 thereof. With the inclusion of the resistive elements, the Wilkinson power divider 81 and the Wilkinson power combiner 83 are lossy (i.e., 2×P2<P1, and P4<2×P3), and a power gain of the conventional radio frequency power amplifier (i.e., a ratio of the power of the radio frequency output signal to the power of the radio frequency input signal) is decreased (i.e., smaller than the predetermined amplification factor). Therefore, the conventional radio frequency power amplifier cannot achieve both the high degree of isolation and the high power gain.